[JENKINS-19494] Call Descriptor#newInstance to instantiate enclosing Publishers.
[JENKINS-19494] Added tests for handling newInstance.
Merge pull request #3 from ikedam/feature/JENKINS-19494_handle_newInstance

[JENKINS-19494] Make flexible publisher to work with publishers with customized Descriptor#newInstance.
[maven-release-plugin] prepare release flexible-publish-0.12